Pseudo-balanced driver

ABSTRACT

A driver for driving laser or a modulator comprises a switch block for switching a current to the laser or modulator and a voltage converter comprising a voltage supply node, an input port for connecting a laser or modulator supply voltage and an output port connected to the switch block. The voltage converter comprises a voltage replica block adapted for generating a pre-defined voltage and is adapted for equalizing DC voltages in the driver by using the pre-defined voltage. The pre-defined voltage approximates the threshold voltage drop of a laser when the driver is connected with a laser or approximates the bias voltage over a modulator when connected with a modulator. The driver comprises a balancer comprising a dummy load connected to the switch block adapted for equalizing voltages in the switch block.

FIELD OF THE INVENTION

The invention relates to the field of drivers for lasers or modulators.More specifically it relates to the field of high-speed drivers forcathode-driven lasers or of high-speed drivers for modulators.

BACKGROUND OF THE INVENTION

A cathode-drive laser driver can be represented in a simplified matteras a current source which can drive a current through a laser. FIG. 1shows schematically such a back terminated laser driver. High-speedcathode-drive laser drivers are typically implemented with a backtermination resistor to move the dominant pole to higher frequencies.

For back-terminated cathode-drive laser drivers the anode voltage of thelaser affects the average laser current and needs to be set at aspecific value in order to minimize this impact.

The back termination resistor R_(t) (resistor 11 in FIG. 1) not onlyreduces the drive efficiency (m) to the laser, but also introduces a DCcurrent path between the supply voltage node of the driver Vdd1 (thevoltage at node 121 in FIG. 1) and the anode voltage node of the laser195 Vdd2 (the voltage at node 191 in FIG. 1).

In the ideal situation the current through the laser only depends on thecurrent from the driver but as there are two supply voltages there is anextra current path between the supply voltage node of the driver Vdd1and the anode voltage node of the laser Vdd2. This extra current dependson the difference between the supply voltages Vdd1 and Vdd2. The currentcan be a negative or a positive current, of which the value is hard topredict or control because of the dependence on R_(t) and on the seriesresistance of the laser R₁ (the laser can be modelled as a voltagesource Vlth and a resistor RI at DC), hence a constraint on the anodevoltage arises that minimizes the extra undesired current through thelaser.

In typical driver designs Vdd2 is higher than Vdd1 with an amount equalto the threshold voltage drop of the laser (Vlth in FIG. 1). The lasersupply voltage is therefore typically 1 to 2 Volt higher. Therefore itis difficult to achieve very low power consumption and this alsocomplicates the external power supply circuitry to power the chip ortransceiver module. Laser technology may for example be operated at 3.3V while transistor technology may typically be operated at 1-2 V.

The graph in FIG. 1 shows the optical power as function of the lasercurrent I_(l). The graph shows the relation between the modulationcurrent I_(m), the bias current I_(b) and the average laser currentĪ_(l).

The relationship between the laser current I_(l) and the driver currentI_(d) is as follows:

$I_{l} = {{I_{d} \cdot m} + \frac{V_{{dd}\; 2} - V_{{dd}\; 1} - V_{lth}}{R_{t +}R_{l}}}$wherein m is the drive efficiency and can be calculated as follows:

$m = {\frac{R_{t}}{R_{t} + R_{l}} < 1}$and wherein the second term in the equation is the extra current throughthe laser. When targeting energy efficient ICs for future data centerinterconnects, it is advantageous that the anode voltage Vdd2 can belowered as much as possible, thereby pursuing single-supply operationfor the laser driver.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide ahigh-speed driver for efficiently driving a cathode-driven laser or amodulator.

The above objective is accomplished by a method and device according tothe present invention.

In a first aspect embodiments of the present invention relate to aback-terminated driver for driving a cathode-driven laser or for drivinga modulator. The driver comprises:

-   -   a switch block for switching a current to either a laser or        either a modulator, wherein the laser or modulator is        connectable with a current node of the switch block so as to        enable current flowing via the current node to the laser or the        modulator,    -   a voltage converter comprising a voltage supply node, an input        port for connecting a laser or modulator supply voltage node and        an output port connected to the switch block, the voltage        converter comprising a voltage replica block which is adapted        for generating a pre-defined voltage, wherein the voltage        converter is adapted for equalizing DC voltages in the driver        during operation of the driver by using the pre-defined voltage,        wherein the pre-defined voltage approximates a threshold voltage        of the laser or a preferred bias voltage over the modulator,    -   a balancer comprising a dummy load connected to the switch block        adapted for equalizing voltages in the switch block. This        balancer has as purpose to minimize the peak-to-peak switching        current through the voltage converter, thereby isolating the        dynamics of the voltage converter from the drivers output        current/voltage at the current node and thus avoiding a        deterioration in high-speed performance of the drivers output        current/voltage.

It is an advantage of embodiments of the present invention that thereplica voltage generated by the replica block allows equalizing DCvoltages in the driver by using the pre-defined voltage. Therebyundesired currents in the switch block can be avoided. When such adriver is used for driving a laser and when the pre-defined voltage isselected such that it approximates the threshold voltage drop of thelaser, an undesired DC current flow between the voltage supply node ofthe laser (Vdd2) and the voltage supply node of the driver (Vdd1) isprevented. It is an advantage of a voltage converter in accordance withembodiments of the present invention that it achieves low-poweroperation by removing the anode voltage constraint. Additionally, thepossibility now arises to choose Vdd2 equal to Vdd1 leveraging anelectro-optic laser transmitter utilizing a single supply voltage. Thiscan greatly simplify the external power supply modules accompanied by adecrease in power dissipation. Power dissipation is further reducedbecause the drive current can maintain its state regardless of the valueof the anode voltage Vdd2, resulting in a minimum required drive currentwhen targeting a certain laser current.

It is an advantage of embodiments of the present invention that at thesame time these drivers are suitable for driving a single ended load.Therefore differences in the current through the voltage convertercaused by switching of the drive current are minimized by the balancer,leading to a better high-speed performance. It is an advantage ofembodiments of the present invention that a higher bandwidth can beachieved than in drivers lacking the back termination resistors. It isan advantage of embodiments of the present invention that no external oron-chip bias tee components are required as such components wouldincrease the cost of the driver and would make the driver less compact.It is an advantage of embodiments of the present invention that theoutput stage is DC coupled. Compared to an AC-coupled configuration asmaller area can be achieved which is advantageous in multi-channeldriver arrays with a restriction on the channel pitch.

When a driver in accordance with embodiments of the present invention isused to drive a modulator it is advantageous that by equalizing DCvoltages in the driver the replica voltage can be applied as biasvoltage over the modulator (i.e. the preferred bias voltage is the biasvoltage of the modulator). When such a driver is used to differentiallydrive a modulator it is an advantage of embodiments of the presentinvention that the amount of bias tee components can be halved and thata bias current source can be avoided, minimizing the amount of area andpower needed to bias the modulator. When such a driver is used tosingle-endedly drive a modulator it is an advantage of embodiments ofthe present invention that no bias tee components are required and thata bias current source can be avoided, minimizing the amount of area andpower needed to bias the modulator.

In some embodiments according to the present invention the switch blockcomprises a differential stage and a corresponding first and second backtermination resistor, wherein a first output pin of the differentialstage is connected to one side of the first back termination resistor atthe current node, and wherein a second output pin of the differentialstage is connected to one side of the second back termination resistorat a second node, and wherein the other sides of both back terminationresistors are electrically connected with a common node, and whereineither a laser or either a modulator is connectable with the currentnode so as to enable current flowing via the current node through thedifferential stage.

In some embodiments according to the present invention the voltageconverter comprises a voltage converter block comprising a first inputnode, a second input node, said voltage supply node and said outputport, wherein said output port is connected to the common node of theswitch block, wherein the voltage replica block comprises said inputport and an output port connected to the first input node of the voltageconverter block and wherein the voltage converter block is adapted forequalizing the DC voltage at the first input node with the DC voltage atthe second input node by controlling the output port.

In some embodiments according to the present invention the second inputnode of the voltage converter block is connected with the common node ofthe switch block when the driver is adapted for driving a cathode-drivenlaser.

In some embodiments according to the present invention the second inputnode of the voltage converter block is adapted for obtaining the averagevoltage at the current node of the switch block when the driver isadapted for driving a modulator. In some embodiments according to thepresent invention the balancer is adapted for equalizing the voltages ofthe current node and of the second node of the switch block.

In some embodiments according to the present invention the voltageconverter block comprises a transistor and an operational amplifier, theoperational amplifier comprising a first input node, a second inputnode, and an output. In some embodiments according to the presentinvention:

-   -   the voltage replica block is connected on one side with the        input port of the voltage converter and on the other side with        the first input node of the operational amplifier,    -   the second input node of the operational amplifier is connected        with the common node of the switch block in case the driver is        adapted for driving a laser,    -   the second input of the operational amplifier is adapted for        obtaining the average voltage at the current node of the switch        block in case the driver is adapted for driving a modulator,    -   the output of the operational amplifier is connected with the        gate of the transistor of the voltage converter block,    -   the transistor of the voltage converter block is connected to        the voltage supply node on one side and to the common node of        the switch block on the other side.

In some embodiments according to the present invention the voltageconverter block comprises an operational amplifier, a current source anda common-mode resistor,

-   -   wherein the voltage replica block is connected on one side with        the input port of the voltage converter and on the other side        with the first input of the operational amplifier,    -   wherein the second input of the operational amplifier is        connected with the common node of the switch block in case the        driver is adapted for driving a laser, or wherein the second        input of the operational amplifier is adapted for obtaining the        average voltage at the current node of the switch block in case        the driver is adapted for driving a modulator,    -   wherein the current source and the common-mode resistor are        connected in series and the common-mode resistor is connected        between the voltage supply node and the common node of the        switch block and the current source between the common node and        ground potential    -   and wherein the output of the operational amplifier is connected        with the current source so as to control the current through the        current source.

It is an advantage of embodiments of the present invention that thevoltage drop between the voltage supply node and the common node can becontrolled by controlling the current through a current source of thevoltage converter.

In some embodiments according to the present invention the voltageconverter block is a switch-mode power supply.

It is an advantage of at least some embodiments of the present inventionthat a more efficient voltage converter block can be realized than incase the converter block is a linear voltage regulator.

In some embodiments according to the present invention the balancercomprises a first and a second input terminal and an output terminaladapted to source current, wherein the first input terminal is connectedto current node of the switch block, and wherein the second inputterminal is connected to second node of the switch block, and whereinthe balancer is adapted to source a current via the output terminal,based on the voltage difference between the first input terminal and thesecond input terminal, to make the load of the switch block symmetrical.

In some embodiments according to the present invention the voltagereplica block is adaptable to a plurality of pre-defined voltages.

It is an advantage that drivers in accordance with at least someembodiments of the present invention can be used to drive a differentlaser with a different laser threshold voltage drop. The conditiontherefore being that the voltage replica block can generate a voltagewhich approximates the laser threshold voltage drop of the laser thedriver is driving. In some embodiments of the present invention thevoltage replica block may be programmable to a plurality of pre-definedvoltages. In embodiments of the present invention the voltage replicablock may comprise an input port which allows to modulate thepre-defined voltage. In some embodiments according to the presentinvention the voltage replica block may comprise a programmableparameter or appropriate temperature dependence to change the copy ofthe threshold voltage drop. It is thereby an advantage that a bettermatch between the threshold voltage drop of the laser and the voltageover the voltage replica block can be achieved.

In a second aspect embodiments of the present invention relate to anopto-electronics transmitter comprising a back-terminated driveraccording embodiments of the present invention and a cathode-drivenlaser, wherein the pre-defined voltage of the voltage replica blockapproximates the threshold voltage drop of the laser. In embodiments ofthe present invention the laser 195 is connected between the voltagesupply node 191 and the current node 119 with its anode towards thevoltage supply node and its cathode towards the current node.

It is an advantage of embodiments of the present invention that thelaser supply voltage can be smaller than the sum of the driver supplyvoltage and the lasers threshold voltage. It is an advantage ofembodiments of the present invention that opto-electronics lasertransmitters according to the present invention can be operated using asingle supply voltage. This simplifies the external power supply modulesand results in a decrease in power dissipation. In addition, the drivecurrent can maintain its state despite a change in anode voltage Vdd2,which would not be the case if the drive current would be adapted basedon the monitored laser current.

In some embodiments according to the present invention theopto-electronics transmitter comprises a driver according to embodimentsof the present invention and a modulator, wherein the top terminal ofthe modulator is AC-coupled with the second node and wherein the bottomterminal of the modulator is connected with the current node of theswitch block, wherein during operation the pre-defined voltage sets thebias voltage of the modulator.

It is an advantage of at least some embodiments of the present inventionthat the voltage replica block can be used to bias a modulator.

It is an advantage of at least some embodiments of the present inventionthat they provide an energy efficient driver for driving a laser or amodulator. It is an advantage of at least some embodiments of thepresent invention that the amount of bias tee components can be reducedor eliminated and that a bias current source can be avoided, minimizingthe amount of area and power needed to bias the modulator.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims. These and other aspects ofthe invention will be apparent from and elucidated with reference to theembodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a back terminated cathode-drive laser driver,as known from prior art.

FIG. 2 shows an opto-electronics transmitter comprising a driver whichis driving a cathode-driven laser in accordance with embodiments of thepresent invention.

FIG. 3 is a graph showing the forward voltage of a laser in function ofthe current through the laser.

FIG. 4 shows a similar driver as in FIG. 2 wherein the switch blockcomprises a transistor pair in accordance with embodiments of thepresent invention.

FIG. 5 shows an opto-electronics transmitter comprising a driver whichis driving a modulator in accordance with embodiments of the presentinvention.

FIG. 6 shows a similar driver as in FIG. 2 wherein the balancer is anidentical laser connected to the dummy branch of the differential pair,in accordance with embodiments of the present invention.

FIG. 7 shows a similar driver as in FIG. 2 illustrating a possibleimplementation of the voltage converter block and of the balancer inaccordance with embodiments of the present invention.

FIG. 8 shows a programmable voltage replica block, more specifically aprogrammable Vbe multiplier, in accordance with an exemplary embodimentof the present invention.

FIG. 9 shows a programmable voltage replica block comprising currentsource with positive temperature coefficient, in accordance with anexemplary embodiment of the present invention.

FIG. 10 show a voltage replica block comprising a reference voltage thatis isolated from the floating terminals of the voltage replica block.

FIG. 11 show a voltage replica block using only the resistor ratio as ascale factor for Vref.

FIG. 12 shows a laser/modulator driver in accordance with embodiments ofthe present invention wherein the driver is configured and connectedwith a cathode-driven laser for driving the laser.

FIG. 13 shows a laser/modulator driver in accordance with embodiments ofthe present invention wherein the driver is configured and connectedwith a modulator for driving the modulator.

FIG. 14 shows an alternative configuration for driving a modulator inaccordance with embodiments of the present invention.

In the different drawings, the same reference signs refer to the same oranalogous elements.

FIG. 15 and FIG. 16 schematically illustrate a driver which is connectedwith a modulator for single-endedly driving the modulator in accordancewith embodiments of the present invention.

Any reference signs in the claims shall not be construed as limiting thescope.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

The terms first, second and the like in the description and in theclaims, are used for distinguishing between similar elements and notnecessarily for describing a sequence, either temporally, spatially, inranking or in any other manner. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

Moreover, the terms top, bottom and the like in the description and theclaims are used for descriptive purposes and not necessarily fordescribing relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other orientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Where in embodiments of the present invention reference is made to“equalizing two voltages”, reference is made to enforcing two voltagesequal to each other.

In a first aspect, the present invention relates to a back-terminateddriver for driving a cathode-driven laser or a modulator. Such a lasermay for example be a vertical-cavity surface emitting laser (VCSEL).

Drivers according to embodiments of the present invention are comprisinga voltage converter 120. The voltage converter comprises a voltagereplica block 124 and a voltage converter block 125.

In embodiments of the present invention the voltage converter 120 tracksthe anode voltage and threshold voltage drop of a laser 195 by insertionof a voltage replica block 124 in the feedback loop that connectsdirectly to the anode voltage node 191. Purpose of the dual voltagetracking is to make the current I_(l) through the laser 195 independentof variations on the lasers anode voltage and forward voltage.

This configuration cancels the current flow I_(extra) between the lasersupply voltage node 191 (voltage Vdd2) and the voltage supply node 121(voltage Vdd1) of the voltage converter. It allows lowering Vdd2 smallerthan Vdd1+Vlth wherein Vlth is the threshold voltage of the laser whichis explained in FIG. 3. In order to isolate the slow dynamics of thevoltage converter block 125 from the high-speed laser current, it isnecessary to reduce the swing of the transient current through theconverter block 125 as much as possible. Therefore a balancer 130 isintroduced in accordance with embodiments of the present invention.

In the example of FIG. 2 the voltage converter 120 is adapted to matchthe voltage at the common node 123 of the switch block 110 with thelaser voltage Vzl. This allows lowering the power consumption of thedriver. The advantage of the voltage converter in this configuration isthat the extra current between Vdd2 and Vdd1 which is equal to

$\frac{V_{{dd}\; 2} - V_{{dd}\; 1} - V_{lth}}{R_{t +}R_{l}}$can be eliminated. As the supply voltage Vdd2 can vary under differentoperating conditions, it is desired that the voltage at the common node123 of the switch block 110 follows these variations. Besides Vdd2, thethreshold voltage drop Vlth of the laser can also slightly change overtemperature or from device to device. It is an advantage of embodimentsof the present invention that also these changes can be tracked by thevoltage replica block 124. In embodiments of the present invention avoltage replica block is inserted between the Vdd2 terminal 191 and thereference voltage terminal (the first input node 126) of the voltageconverter block 125.

In embodiments of the present invention the voltage of the voltagereplica block 124 voltage can be modified by providing a control inputterminal 128 to cover a broad range of laser applications. Inembodiments of the present invention the voltage replica block 124comprises a dedicated control input terminal 128. It is thereby anadvantage that this allows interfacing the driver 100 to a broad rangeof lasers or provide calibration over temperature and after fabrication.

In the exemplary embodiment illustrated in FIG. 2 the voltage converterblock 125 will adjust its voltage drop until Vz=Vdd2−Vlr wherein Vlr isthe voltage over the voltage replica block 124 and Vz is the voltage atthe common node 123 of the switch block 110. The extra current isthereby minimized if Vlr approximates the threshold voltage drop Vlth ofthe laser. In embodiments of the present invention the voltage converterblock 125 can be implemented as a linear regulator or as a switched-modepower supply. An example of a linear regulator is illustrated in FIG. 7.

FIG. 3 is a graph showing the forward voltage of a laser in function ofthe current through the laser. The forward voltage refers to the totalvoltage drop over the laser. This also includes the resistive voltagedrop from R_(l).

Where in embodiments according to the present invention reference ismade to the threshold voltage drop Vlth, reference is made to thevoltage where the laser is starting to emit light. This voltage can beobtained by extracting it from the measured curve via a linear fittedmodel as shown in FIG. 3. The graph shows the forward voltage infunction of the laser current. The dashed line shows the linear fittedmodel which can be expressed as:V _(l) =V _(lth) +I _(l) ·R _(l)

In embodiments of the present invention the pre-defined voltage of thevoltage replica block approximates the threshold voltage drop within arange of +/−5%, or even less. In embodiments of the present inventionthe voltage converter 120 is adapted for approximating the voltage atthe common node 123 to the internal voltage of the laser Vzl (see FIG.2). In embodiments of the present invention approximating the thresholdvoltage drop value means reaching it within a range of +/−5%.

When driving a single ended load as in FIG. 2, the voltage convertercreates a node voltage Vz at the common node 123 that is lower than Vdd1(the voltage at the voltage supply node 121).

The extra current between Vdd2 (the laser supply voltage at node 191)and Vdd1 becomes zero when Vdd2 equals the sum of Vlth and Vz and thusallows lowering the anode voltage independent of Vdd1.

In embodiments of the present invention the switch block 110 is adifferential pair/driver that implements back termination resistors totarget high-speed operation. Low-speed drivers avoiding these backtermination resistors do not need the anode and forward voltage trackingloop as there is no undesired current flow between the two supplyterminals.

When driving a single ended load as in FIG. 2, but without the presenceof the balancer, the transient current Iz through the voltage converterblock 125 equals the transient current through the laser due to theasymmetric load and has a peak-to-peak swing of m·Im. The dynamics ofthe voltage converter block 125 are significantly slower than thedifferential driver 116. Hence, the settling time of the converter block125 will distort the eye diagram for patterns with long strings ofConsecutive Identical Digits (CID) and results in increased intersymbolinterference (ISI), noticeable in graph g of FIG. 2. The ISI leads to adeteriorated eye height and eye width. This introduces a power penaltyat the receiver side and makes the design of the Clock and Data Recovery(CDR) less robust.

Therefore drivers according to the present invention are comprising abalancer to preserve the signal quality in the optical link by balancingthe output stage. Thereby the output stage is the block that drives thelaser. It may comprise the switch block 110, the bias converter 150, andthe voltage converter 120. The balancer minimizes the transient currentthrough the voltage converter. In embodiments of the present inventionthe balancer 130 is a three-terminal control circuit block that sensesthe voltages of both branches of the differential pair at the currentnode 119 and at the second node 117, and sources a current Id into thedummy branch to force the voltage at the second node 117 equal to thevoltage at the current node 119. Consequently the differential pair seesa symmetrical load at DC. Because it is impossible to perfectly matchboth impedances, the AC input impedance of the balancing circuit willdiffer from that of the laser, hence the name ‘pseudo-balanced’.

Such a balancer 130 may comprise a dummy load connected to the switchblock 110 adapted for enforcing the voltages at the current node 119 andat the second node 117 equal to each other, with the primary purpose tominimize the peak-to-peak switching current through voltage converterblock 125, thereby isolating the dynamics of the voltage converter block125 from the drivers output current/voltage e and thus avoiding adeterioration in high-speed performance of the drivers outputcurrent/voltage e (visually explained by graphs f and g in FIG. 2; graphf showing the undistorted eye diagram and graph g showing thedeteriorated eye diagram).

It is an advantage of balancer 130 according to the present inventionthat they can also improve high-speed performance in back terminateddifferential drivers without integrated voltage converter since anasymmetric load can result in non-linear effects such as a pulse-widthdistorted output current.

The drive efficiency to the dummy load will also deviate from m and istherefore referred to as m′. Without the balancer, the peak-to-peaktransient current Iz through the voltage converter block 125, is equalto (m·Im). It is an advantage of embodiments of the present inventionthat by introducing the balancer the value of the peak-to-peak transientcurrent Iz through the voltage converter block 125 is reduced with afactor (1−m′/m). In the ideal case where m′=m, Iz remains at a constantlevel resulting in a laser current I_(l) free from ISI introduced bysettling time effects, as can be observed in the eye diagram f,illustrated in FIG. 2. In FIG. 2 the following peak-to-peak transientcurrents and the following base currents apply:

For signal a: m·Im·(1−m′/m); (2−m−m′)·Ib+(1−m)·Im

For signal b: (1−m′)·Im; (1−m′)·Ib

For signal c: (1−m)·Im; (1−m)·Ib

For signal d: m′·Im; −(1−m′)·Ib

For signal e: m·Im; m·Ib

In the example of FIG. 4, the first transistor 112 and the secondtransistor 113 together with the first back termination resistor 114 andthe second back termination resistor 115 constitute the differentialpair switching a current I_(m) to the single-ended laser. In embodimentsaccording to the present invention the modulation current source I_(m)(140) can be replaced with a resistor to further lower Vtail andincrease the headroom of the first and second transistor 112, 113 ifnecessary. Current source I_(b) (150) can also be omitted if no constantbiasing of the laser is required.

In embodiments of the present invention a common-mode feedback loop maybe added in the preceding stage to set Vtail to increase the headroom ofthe first and second transistor 112, 113.

For FIG. 4 the same signal levels apply as for FIG. 2. Also the same eyediagram can be obtained.

In embodiments of the present invention, such as illustrated in FIG. 4,the voltage at the common node 123 may be equalized with the voltage atthe first input node 126. In that case the voltage at the common node123 will approximate the node voltage Vzl inside the laser. The voltageat the current node 119 equals Vzl minus a voltage drop across laserresistor RI.

FIG. 5 shows an opto-electronics transmitter 800 comprising a driver 100and a modulator 810 which can be differentially driven by the driver100. A modulator typically requires a modulation voltage Vs and a biasvoltage to modulate the optical output power.

The modulator can be single-ended DC-coupled to the driver, wherein thebottom terminal is connected to the current node 119 of the switch block110 and the top terminal to the common node 123 of the switch block,effectively placing the modulator in parallel with resistor 114. Withoutthe presence of a voltage converter, node 123 corresponds to supply node121. This configuration would however result in an increased powerconsumption to generate the bias voltage.

The power consumption is reduced by AC-coupling both nodes (the secondnode 117 and current node 119 of the switch block 110), but thisrequires two bias voltages and two bias tees. Multiple combinations ofthese configurations are possible but all suffer from previouslymentioned drawbacks.

In embodiments of the present invention the top terminal of themodulator 810 is AC-coupled to the second node 117 of the switch block110 (e.g. over capacitor 830) while also connected to the modulatorsupply voltage node 191 and the bottom terminal of the modulator 810 isDC-coupled to the current node 119 of the switch block 110. In theexemplary embodiment of FIG. 5 the top terminal is connected (throughe.g. an inductor 820) to the supply voltage Vdd2 (at modulator supplyvoltage node 191). In this embodiment of the present invention thevoltage converter 120 is adapted for equalizing DC voltages in thedriver 110 during operation of the driver by using the pre-definedvoltage. Therefore the average voltage of the current node 119 of theswitch block 110 is used as the second input node 127 of the voltageconverter block 125. The advantage thereof is that the replica voltageV_(lr) of the voltage replica block 124 directly sets the bias voltageof the modulator 810. This can be illustrated by the following formulas:V _(l) =Vdd2− V _(y)V _(y) =Vdd2−V _(lr)wherein V_(y) is the voltage at the current node 119 of the switch block110, and wherein V_(l) is the voltage over the modulator 810. From thetwo previous formulas it follows that:V ₁ =V _(lr)

In embodiments of the present invention the pre-defined voltage of thevoltage replica block can approximate the bias voltage within a range of+/−1% or less depending on the gain and the voltage offset in thefeedback loop of voltage converter 123.

It is an advantage of this exemplary embodiment of the present inventionthat this technique only requires one bias tee while still acquiring aprecise control of the bias voltage. It is an advantage of thisexemplary embodiment that no bias current is needed to generate a biasvoltage thereby saving static power.

In the exemplary embodiment illustrated in FIG. 5 a balancer 130 isconnected to the switch block 110. The advantage thereof being that ifthe modulator 810, such as an electroabsorption modulator, sources acurrent I_(l) to the driver because of the absorbed photons and if thiscreates an unbalanced output stage where the voltage Vx at the secondnode 117 of the switch block differs from the voltage Vy at the currentnode 119 of the switch block, the balancer will source a current I_(d)to make the load symmetrical again.

The graph in FIG. 5 shows the output power of the modulator in anarbitrary function of the applied voltage. It shows the bias voltage Vlrand a modulation voltage Vs.

FIG. 6 shows a driver in accordance with embodiments of the presentinvention wherein the balancer is an identical laser connected to thedummy branch of the differential pair. Through current sources 950, 150the base current Ib is drawn. Through current source 140 the modulationcurrent Im is drawn. For FIG. 6 the same signal levels apply as for FIG.2, except for the base current of signal d that now equals m′·Ib. Alsothe same eye diagram can be obtained, although in a less efficient waysince an extra laser is necessary of which the optical output is notused.

FIG. 7 shows a driver 100 for driving a laser 195 in accordance withembodiments of the present invention. The driver is similar as thedriver in FIG. 2 and FIG. 4. This exemplary embodiment shows a possibleimplementation of the voltage converter block 125 and of the balancer130.

The driver 100 is balanced by applying a dummy load to the left branchof the transistor pair 112, 113. The dummy load (i.e. the balancer 130)comprises a first input terminal 131, a second input terminal 132 and anoutput terminal 133. The balancer moreover comprises an operationalamplifier 1040 and a transistor 1050. The first and second inputterminal 131, 132 are the inputs of the operational amplifier. Theoutput of the operational amplifier is connected with the gate of thetransistor 1050.

The first terminal 131 is connected with the current node 119 of theswitch block 110 and the second terminal 132 as well as the outputterminal are connected with the second node 117 of the switch block 110.The output terminal is connected over a resistor 1030 with the emitter(source if a FET is used) of the transistor 1050. The collector (drainin case of FET) of the transistor 1050 is connected with the voltagesupply node 121.

The operational amplifier 1040 and the transistor 1050 are configured ina feedback loop so as to equalize the voltage of the current node 119and the voltage of the second node 117 of the switch block.

The current source 1060 is connected with the emitter of the transistor1050. This current source guarantees a current through the transistor1050, such that sufficient negative feedback is present under alloperating conditions, by adding it up to the base current of signal dwhich is negative and proportional to Ib and (1−m′). The current source1060 is a scaled version of the bias current with a scale factor 1/kthat should be greater than or equal to (1−m′). Although in practice,this constraint may vary and should be evaluated carefully taking intoaccount all non-idealities.

In embodiments of the present invention the input transistors of thebalancing amplifier 1040 are designed small in area in order to limitthe added parasitic capacitance at output node Vy (the current node119). However, reducing the transistor size enlarges the variance overprocess corners of the offset voltage at the inputs of amplifier 1040.This imposes a slight imbalance between the two branches of thedifferential pair which translates into a higher transient currentthrough the voltage converter block 125. Typically, the parasiticcapacitance introduced by the bias current source Ib and the bond paddominate the input capacitance of amplifier 1040. If necessary, theinput transistors of the amplifier 1040 can still be dimensioned smallif the undesired input offset voltage is trimmed or calibratedpost-production.

The voltage converter block 125, in the example of FIG. 7 is a linearvoltage regulator. It comprises an operational amplifier 1010 and atransistor 1020. The voltage replica block 124 is connected with thefirst input of the operational amplifier 126. The second input 127 ofthe operational amplifier 1010 is connected with the common node 123 ofthe switch block 110. The output of the operational amplifier 1010 isconnected with the gate of the transistor 1020 of the voltage converterblock 125. The transistor 1020 of the voltage converter block 125 isconnected to the voltage supply node 121 on one side and to the commonnode 123 of the switch block 110 on the other side.

In embodiments according to the present invention the transistor 1020can be replaced with a current source controlled by the operationalamplifier 1010, creating a voltage drop across a common-mode resistorconnected between Vdd1 (voltage supply node 121) and Vz (common node123).

In embodiments according to the present invention the converter block125 may also be a switched-mode power supply.

The transistors present in a driver, in accordance with embodiments ofthe present invention, can be MOSFET and/or BJT depending on technologyand available headroom.

Multiple differential pairs can connect to nodes Vx (117) and Vy (119)to enable feed-forward equalization topologies or multi-levelmodulation.

FIG. 8 shows a voltage replica block 124 in accordance with an exemplaryembodiment of the present invention. In embodiments of the presentinvention the input 851 and the output 852 of the voltage replica blockare floating terminals over which a voltage drop is created. Preferably,the voltage drop can be made programmable and/or follow a certaintemperature dependence. A possible implementation is a programmableVbe-multiplier using bipolar junction transistors as shown in FIG. 8.The voltage drop of the replica circuit equals Vce (thecollector-emitter voltage) and its expression is stated below. Assumingthat the current gain factor β of the transistor Q0 is infinite (nocurrent is flowing into the base of the transistor), Vce solely dependson the base-emitter voltage of the transistor Vbe and the resistorratio. With this topology, Vce is lower bounded to Vbe and upper boundedto the breakdown voltage of transistor Q0.

${{{{V_{ce} = {V_{be} \cdot \left( {1 + \frac{R_{a}}{R_{b}}} \right)}}}_{\beta = \infty} = {V_{be} \cdot \left( {1 + {R_{a} \cdot {\sum\limits_{i = 0}^{N - 1}\frac{1}{b_{i} \cdot R_{i}}}}} \right)}}}_{\beta = \infty}$

The voltage drop Vce can be made programmable by inserting switches inseries with resistors R₀ to R_(N-1). controlled with bits b₀ to b_(N-1).These bits can be derived from a lookup table (LUT) that e.g. relatesthe bit word to the ambient temperature to track the lasers temperaturedependence. The LUT can also store bit words related to types of laserswith a different threshold voltage.

In embodiments according to the present invention the lasers temperaturedependence can also be tracked by the intrinsic characteristics of thereplica circuit itself. Bipolar junction transistors are characterizedby a Vbe that has a temperature coefficient (TC) of approximately −2mV/K. This makes the TC of the Vbe multiplier equal to (1+Ra/Rb)*(−2mV/K). This TC can be altered by using a reference current source Irefthat has a positive or negative TC since Vbe also depends on the currentthat flows through the transistor. FIG. 9 shows a programmable voltagereplica block 124, more specifically a programmable Vbe multiplier, inaccordance with embodiments of the present invention. The current sourceis in this case a current source with a positive TC. This current sourceis also referred to as a proportional to absolute temperate (PTAT)current source. Iref can be expressed as:

${Iref} = \frac{{{Vdd}\; 1} - {2 \cdot {Vbe}}}{Rc}$

Since Vbe decreases with temperature, Iref will therefore rise withtemperature and partially counteract the TC of the Vbe multiplier inFIG. 8.

Note that the reference voltage Vbe of the Vbe multiplier is sensitiveto process variations, although much less than the gate-source voltageof a FET used in a Vgs multiplier, and perhaps can have a TC that doesnot match with the TC of the laser.

An alternative is to use a precise and stable voltage generator, such asa bandgap reference, however these circuits don't have two floatingterminals since they are typically referenced to ground.

The voltage replica circuit in FIG. 10 solves this issue by utilizingvoltage-to-current conversions in a feedback loop. Amplifier A1 drivescontrol terminal Vg of first current mirror 853 until the voltage acrossresistor R1 equals the stable reference voltage Vref, thus creating acurrent |1=Vref/R1. The first current mirror 853 scales 11 with a factor(mirror ratio) m/n, which can be a programmable parameter implementedwith bit-controlled switches, while the second current mirror 854sources the scaled current to a second resistor R2 with a resistanceequal to R1. The voltage drop across R2 then equals m/n*Vref andcorresponds to the voltage drop Vce. Assuming m/n=1, Vref can be copiedto the output within the range of +/−1% as current mirrors and resistorscan be designed and layout to match very well.

Note that the scale factor m/n can be distributed between the twocurrent mirrors or included in the resistor ratio R2/R1 and can be madesmaller or larger than unity resulting in a Vce smaller or larger thanVref. Compared to the Vbe multiplier, the replica voltage Vce is notupper bounded by e.g. a breakdown voltage.

In embodiments according to the present invention the first and secondcurrent mirrors 853, 854, as illustrated in FIG. 10, can be avoided byusing the topology shown in FIG. 11. In this exemplary embodiment of thepresent invention amplifier A1 drives transistor M1 until the voltageacross the bottom resistor Rb (in FIG. 11 this corresponds with thecombination of the parallel resistors in series with the bit controlledswitches) equals Vref, thereby generating a current Vref/Rb. Thiscurrent also flows through resistor Ra, creating a voltage drop equal toVref*Ra/Rb and corresponds to the voltage replica Vce. Similar to theVbe multiplier from FIG. 8, resistor Rb can be made programmable byincluding bit-controlled switches in series with the bottom resistors.This topology will consume less area than the topology from FIG. 10because the current mirrors 853, 854 are absent. The voltage replicablock illustrated in FIG. 10 is, however, less prone to processvariations resulting in a scale factor which is less process sensitive.

In summary, it is an advantage of a driver 100 in accordance withembodiments of the present invention that it can provide an accurate andstable biasing of a load while simultaneously minimizing the powerconsumption. This can moreover be achieved at a high data rate (e.g. 40Gb/s). It is moreover advantageous that only a minimum of bias tee/RLCinterface elements are required for interfacing with the load (exampleswill be illustrated using FIG. 11, FIG. 12, and FIG. 13). In embodimentsof the present invention such a driver may even lead to a single-supplyvoltage operation of the laser/modulator driver. In embodiments of thepresent invention the load is either a cathode-driven laser or either amodulator. In both cases, using a driver in accordance with embodimentsof the present invention will result in a reduced power consumption.

Power consumption is minimized by firstly reducing waste current throughthe load, e.g. between the voltage supply Vdd2 at the load supply node191 and the voltage supply of the driver Vdd1 at the voltage supply node121, and secondly by making the drive current independent of the loadsupply voltage Vdd2, thereby simplifying the design of the precedingstage.

FIGS. 12, 13, and 14 illustrate how a driver 100, in accordance withembodiments of the present invention, can be used and configured fordriving a laser 195 or for driving a modulator 810. Driving differenttypes of loads can for example be driven by connecting the loaddifferently to the driver 100 and by switching a different feedbackinput to the voltage converter at the second input node 127.

Conceptually, it is possible to use the same circuit for both scenarios,as shown in FIGS. 12, 13 and 14 by making the DC voltages that areequalized in the driver configurable. This may for example be achievedusing switches 171, 172.

In these examples the second input node 127 of the voltage converterblock 125 (i.e. the feedback input) being either the voltage Vz at thecommon node 123 (when driving a laser) or the average of the voltage Vyat the current node 119 of the switch block 110 (when driving amodulator). Selecting a different input for the second input node may beachieved using a first switch 171 for selecting the average of thevoltage Vy, and a second switch 172 for selecting the voltage Vz. Thereplica voltage also needs to be adapted representing either a laserforward threshold voltage or either a bias voltage of the modulator.

The driver may for example comprise bondpads for differently connectingan external load. In the examples of FIGS. 11, and 12 an inductor 184 isconnected between the input port 122 of the voltage converter 120 and acapacitor 185. The capacitor 185 is connected with the second node 117of the switch block 110. A first bondpad 181 corresponds with the inputport 122 of the voltage converter 120. A second bondpad 182 is connectedwith the interconnection between the capacitor 185 and the inductor 184.A third bondpad 183 is connected with the current node 119 of the switchblock. In FIG. 13 instead of an inductor 184, a resistor 186 is presentbetween the first bondpad 181 and the second bondpad 182.

When driving a laser, the voltage converter 120 is adapted forequalizing the voltage at the input port 126 of the voltage converterblock 125 with the voltage at the output port 123 by using thepre-defined voltage subtracted from the voltage at input port 122wherein the pre-defined voltage approximates a threshold voltage of thelaser. In that case the laser may be connected as illustrated in FIG.12. The anode of the laser together with the load supply voltage Vdd2 isconnected with the input port 122. This may be achieved by connectingthe anode of the laser and the load supply voltage with the firstbondpad 181. In the example the second bond pad 182 is left floatinghaving the disadvantage of a less denser integration of laser drivers ina certain area. The cathode of the laser is connected with the currentnode 119 of the switch block 110. This may be achieved by connecting thecathode of the laser with the third bondpad 183.

In case of driving a modulator the voltage converter 120 is adapted forequalizing the voltage at the input port 126 of the voltage converterblock 125 with the average voltage of current node 119, by using thepre-defined voltage subtracted from the voltage at input port 122wherein the predefined voltage approximates a bias voltage of themodulator. In case of driving a modulator, the bias current Ib generatedby the current source 150 is not necessary and can be disabled. Themodulator 810 is on one side connected with the node connecting theinductor 184 and the capacitor 185, and is on the other side connectedwith the current node 119 of the switch block. In the example of FIG. 13the modulator 810 is connected between the second bondpad 182 and thethird bondpad 183 while the load supply voltage node 191 is connectedwith the first bondpad 181.

In embodiments of the present invention the driver comprises a laser ora modulator. In case of a laser, the laser is connected between thecurrent node 119 and the supply voltage node 191 such that it is cathodedriven. In case of driving a modulator differentially, the modulator isconnected with the current node 119 and second node 117 and with thesupply voltage node 191 through a bias tee. In case of driving amodulator single-endedly, the modulator is connected with the currentnode 119 and with the supply voltage node 191. In both cases the voltageconverter 120 is adapted for equalizing DC voltages in the driver duringoperation of the driver 100 by using the pre-defined voltage.

The voltage converter 120 is mainly dealing with the accurate and stablebiasing part. To pursue high data rates, the insertion of a balancer 130is required.

In embodiments of the present invention the current through node 119provided by current source 140 and/or current source 150 can be kept toa minimum to bias a laser or a modulator. In embodiments of the presentinvention the balancer sources a current into second node 117 in orderto balance the load and improve high-speed performance.

These examples illustrate the way the driver can be connected to a loaddepending on the load type. The invention, however, is not limited tothe configurations illustrated in FIG. 12 to FIG. 14. The circuitry maybe optimized depending on the load. For example the modulation currentIm generated by the modulation current source 140, the terminationresistors 114, 115, the driver supply voltage Vdd1 at the voltage supplynode 121 and load supply voltage Vdd2 at the voltage supply node 191 cangreatly differ for a laser and a modulator.

FIG. 12 and FIG. 13 assume an LC-biasing scheme 184, 185 for themodulator 810, however, in some cases the area required by the inductor184 is not feasible. An alternative therefore would be to use an RC biastee between the load supply node 191 and the second node 117. Someoptical modulators, e.g. an electro-absorption modulator, source aphoton current I_(l) depending on the intensity of the incoming light.The bias resistor 186 introduces then an undesired voltage dropresulting from the photon current, thereby affecting the bias voltageacross the modulator. This issue is solved by the circuit in FIG. 14. Inthis exemplary embodiment of the present invention an accurate biasvoltage independent of photon current is provided. A k-scaled copy 187of the bias resistor 186 is connected in series with the replica voltagesource 124. Current is drawn from the replica series connection with ak-scaled copy of the balance current Id, that under correct operationand assuming Ib is disabled, equals photon current I_(l). In thisexample the k-scaled copy of the balance current Id is drawn by means ofa current source 188.

FIG. 15 and FIG. 16 schematically illustrate another exemplaryembodiment of the present invention wherein the driver is connected witha modulator for single-endedly driving the modulator.

In the example of FIG. 15 the driver corresponds with the driverillustrated in FIG. 5. In this example the modulator 810 is connectedbetween the current node 119 and the supply voltage node 191.

In FIG. 16 the driver has the same configuration as the drivers in FIGS.12 to 14 comprising the first 181, second 182 and third 183 bondpads. InFIG. 16 the modulator 810 is connected between the first 181 and thirdbondpad 183. The first bondpad is connected with the load supply voltagenode 191.

The invention claimed is:
 1. A driver for driving a laser or amodulator, the driver comprising: a switch block for switching a currentto the laser or modulator, wherein the laser or modulator is connectablewith a current node of the switch block so as to enable current flowingvia the current node to the laser or the modulator, a voltage convertercomprising a voltage supply node, an input port for connecting a laseror modulator supply voltage node and an output port connected to theswitch block, the voltage converter comprising a voltage replica blockwhich is adapted for generating a pre-defined voltage, wherein thevoltage converter is adapted for equalizing DC voltages in the driverduring operation of the driver by using the pre-defined voltage, whereinthe pre-defined voltage approximates a threshold voltage of the laser ora preferred bias voltage over the modulator, a balancer comprising adummy load connected to the switch block adapted for equalizing voltagesin the switch block.
 2. The driver according to claim 1, wherein theswitch block comprises a differential stage and a corresponding firstand second back termination resistor, wherein a first output pin of thedifferential stage is connected to one side of the first backtermination resistor at the current node, and wherein a second outputpin of the differential stage is connected to one side of the secondback termination resistor at a second node, and wherein the other sidesof both back termination resistors are electrically connected with acommon node, and wherein the laser or modulator is connectable with thecurrent node so as to enable current flowing via the current nodethrough the differential stage.
 3. The driver according to claim 1,wherein the voltage converter comprises a voltage converter blockcomprising a first input node, a second input node, said voltage supplynode and said output port, wherein said output port is connected to thecommon node of the switch block, wherein the voltage replica blockcomprises said input port and an output port connected to the firstinput node of the voltage converter block and wherein the voltageconverter block is adapted for equalizing the DC voltage at the firstinput node with the DC voltage at the second input node by controllingthe output port.
 4. The driver according to claim 3, wherein the secondinput node of the voltage converter block is connected with the commonnode of the switch block when the driver is adapted for driving a laser,or wherein the second input node of the voltage converter block isadapted for obtaining the average voltage at the current node of theswitch block when the driver is adapted for driving a modulator, whereinthe balancer is adapted for equalizing the voltages of the current nodeand of the second node of the switch block.
 5. The driver according toclaim 3, wherein the voltage converter block comprises a transistor andan operational amplifier, the operational amplifier comprising a firstinput node, a second input node, and an output, wherein the voltagereplica block is connected on one side with the input port of thevoltage converter and on the other side with the first input node of theoperational amplifier, wherein the second input node of the operationalamplifier is connected with the common node of the switch block in casethe driver is adapted for driving a laser, or wherein the second inputof the operational amplifier is adapted for obtaining the averagevoltage at the current node of the switch block in case the driver isadapted for driving a modulator, wherein the output of the operationalamplifier is connected with the gate of the transistor of the voltageconverter block, and wherein the transistor of the voltage converterblock is connected to the voltage supply node on one side and to thecommon node of the switch block on the other side.
 6. The driveraccording to claim 3, wherein the voltage converter block comprises anoperational amplifier, a current source and a common-mode resistor,wherein the voltage replica block is connected on one side with theinput port of the voltage converter and on the other side with the firstinput of the operational amplifier, wherein the second input of theoperational amplifier is connected with the common node of the switchblock in case the driver is adapted for driving a laser, or wherein thesecond input of the operational amplifier is adapted for obtaining theaverage voltage at the current node of the switch block in case thedriver is adapted for driving a modulator, wherein the current sourceand the common-mode resistor are connected in series and the common-moderesistor is connected between the voltage supply node and the commonnode of the switch block and the current source between the common nodeand ground potential, and wherein the output of the operationalamplifier is connected with the current source so as to control thecurrent through the current source.
 7. The driver according to claim 3wherein the voltage converter block is a switch-mode power supply. 8.The driver according to claim 1 wherein the balancer comprises a firstand a second input terminal and an output terminal adapted to sourcecurrent, wherein the first input terminal is connected to current nodeof the switch block, and wherein the second input terminal is connectedto second node of the switch block, and wherein the balancer is adaptedto source a current via the output terminal, based on the voltagedifference between the first input terminal and the second inputterminal, to make the load symmetrical.
 9. The driver according to claim1, wherein the voltage replica block is adaptable to a plurality ofpre-defined voltages.
 10. An opto-electronics transmitter, theopto-electronics transmitter comprising a driver according to claim 1and a laser, wherein the pre-defined voltage of the voltage replicablock approximates the threshold voltage drop of the laser.
 11. Anopto-electronics transmitter, the opto-electronics transmittercomprising a driver according to claim 1 and a modulator, wherein thetop terminal of the modulator is AC-coupled with the second node whilealso DC-coupled to the modulator supply voltage node and wherein thebottom terminal of the modulator is connected with the current node ofthe switch block, wherein during operation the pre-defined voltage setsthe bias voltage of the modulator.